Standard 8: Reaction Rate

ChemistryMs. Siddall

Reaction rate = speed of reaction• Example

•An explosion happens quickly•Rusting happens slowly

8a: Measuring reaction rate

• Reaction rate = change in concentration time

•∆[reactants] or ∆[products] time time

• ∆ = change• [ ] = concentration

Summary 1

• What is the rate of reaction when 5 moles of reactant is used in 10 seconds?

Reversible reaction: X Y[X]

[Y]

time

X Y

Y X

timerate

concentration

•As concentration decreases - Rate decreases•As concentration increases - Rate increases•When concentration is constant - Rate is constant

Summary 2

• Describe how rate changes as concentration changes.

Reaction mechanics

• A reaction can only occur if:1. Molecules collide2. Molecules collide with enough energy3. Molecules collide with the correct

orientation

Molecules must hit hard enough in just the right place

Summary 3

• What are the conditions at the molecular level for a reaction to occur?

There are 4 factors that affect the rate of a reaction:

1. Temperature:Increasing temperature = more energy= more collisions = more reactions

8b: Factors affecting reaction rate

Summary 4

• Why do changes in temperature affect reaction rate?

2. Concentration:Increasing concentration = more particles = more collisions = more reactions

low concentration high concentration

Summary 5

• What happens to the number of collisions when there is an increase in concentration?

3. Surface Area:Increasing surface area = more places to collide = more collisions = more reactions

Reaction occurs only at the surface

More surface area = More chance to react

Summary 6

• 1. Which has more surface area? • A 1g sugar cube or 1g of granulated

sugar

• 2. Which would react faster?

4. Pressure (gases only):Increasing pressure = particles closer together = more collisions = more reactions

Increased pressure forces particles

together

low pressure higher pressure

Summary 7

• 1. How would you increase the rate of a gas reaction?

• 2. Why does increasing pressure NOT help increase the rate of a liquid reaction?

en

erg

y

time

reactants

products

Activation energy

Reaction pathway• Reactions are not always

spontaneous• Activation energy = energy

needed for a reaction to happen

Summary 8

• Paper burns in the presence of oxygen. Explain why this paper does not burst into flames.

Catalyst• A catalyst increases the rate of a

reaction by lowering ‘activation energy’ • A catalyst is not used up in the reaction• Example: a catalytic converter allows

CO to react more easily with O2 to make CO2

standard 8c: catalyst

Reaction pathwaycopy diagram on page 547

Summary 9

1. In terms of ‘activation energy’, how does a catalyst increase the rate of reaction?

2. Does a catalyst affect the energy of reactants or products?

Standard 8d: the definition and role of activation energy in a chemical reaction.

• Reactants must form an ‘activated transition complex’

• This requires energy = activation energy

Honors.

en

erg

y

time

reactants

products

Reaction with catalyst

Activated transition complex

Summary 10

• 1. Which has a higher energy: • Reactants or Products?

• 2. How does the catalyst affect the energy of the activated transition complex?